Field of the Invention
The invention relates to a monolithically integrated active free-running element for use in a current switch device with inductive loading, of the kind that can be used in secondary switching regulators. In particular, it relates to a largely monolithically integratable current switch device for such a switching regulator.
As described in a book entitled "Halbleiterschaltungstechnik" [Semiconductor Circuitry], by Tietze and Schenk, 9th Edition, 1991, Chapter 18.6, "Sekundargetaktete Schaltregler" [Secondary Switching Regulators], pp. 563 ff., and in particular Chapter 18.6.1, pp. 563-566, depending on their structure, secondary switching regulators include a current switch which, upon being controlled, typically by a closed and open-loop control circuit whenever the current switch periodically switches, switch an inductance either between the input voltage and the reference potential, or between the output voltage and the reference potential, or between the input voltage and the output voltage. So far, such current switches have been achieved by means of an electronic power switch and a so-called recovery or free-running diode. Either a thyristor or a bipolar or field-effect transistor is provided in such a case as a power switch. In that case, however, the efficiency of switched-mode power supplies depends on the forward voltage drop of the recovery diode. Particularly in switched-mode power supplies with significant step-down voltage conversion, that is so-called step-down converters, the efficiency of the switched-mode power supply depends very strongly on the forward voltage drop of a recovery diode. In switched-mode power supplies with a low output voltage, such as 5 V for logic power supplies, that influence is especially pronounced. Previously, the efficiency of such switched-mode power supplies could be improved only by using Schottky diodes, which are known to have a forward voltage drop of approximately 0.4 V, in contrast to the usual silicon diodes having a forward voltage drop of approximately 0.65 to 0.8 V. However, the integration of Schottky diodes requires a great amount of surface area and therefore is expensive, and provision is not made for such integration in all semiconductor technologies. Such switched-mode power supplies therefore require not only the power switch as a discrete component, but the recovery diode as well.
It is accordingly an object of the invention to provide a monolithically integratable free-running element, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which has a low forward voltage.